fix 27-day check

lib/core.js

@@ -352,7 +352,7 @@ function getOrCreateDomainCertificate(args, defaults, handlers) {
       // no certs, seems like a good time to get some
       return getCertificateAsync(args, defaults, handlers);
     }
-    else if (certs.issuedAt > (27 * 24 * 60 * 60 * 1000)) {
+    else if ((Date.now() - certs.issuedAt) > (27 * 24 * 60 * 60 * 1000)) {
       // cert is at least 27 days old we can renew that
       return getCertificateAsync(args, defaults, handlers);
     }